@Slereah Not with that attitude, you can't. =)

NB: Electric fields and magnetic fields cannot really form a vortex since they are always orthogonal to each other

so no way can the vectors superimpose in such a way to give a spiral

Having said that, it will be interesting to see what the field in a changing constant magnetic field (which by faraday induction, will induce a varying circular electric field) with a point charge held in place is like. Perhaps it will give us a vortex shaped electric field

bem.fi/book/12/12.htm

Hi, everybody.

having said that x2. There do exists optical vortices: photonics.com/a52139/Optical_Vortex_Beams_Emit_on_Silicon

@dmckee Come now. Surely you understand it, no?

that's one big issue of the public: Too immersed in technobabble

I'm still not at all sure what that one set of field lines was supposed to represent.

me neither, other than someone is trying to superimpose vectors of two different quantities

Right.

In theory, that configuration can be realised if we somehow can immobilise a point charge and then subject that to a varying magnetic field pointing to the z direction, so that the induced electric field adds to the radial field of the charge, but that will just be a spiral shape electric field, not an electromagnetic field

I can appreciate the suggestion that thinking of "Maxwell's equations" as synonymous with "Maxwell's unification of electric and magnetic fields" is anachronistic

Maxwell did work with 'vortex' models of electric/magnetic phenomena in order to gain insight into them

I have not read EM history much, thus I cannot comment much

But, uh...if Maxwell's equations aren't how Maxwell would have understood unification, then so much the worse for him

We do have Maxwell's equations, and they work very very well within their domain

I think back in those days magnetic charge is a thing in EM models?

I'm not sure that's true either.

My main thing is that the appeal to Maxwell as more authoritative than Maxwell's equations just seems strange to me.

I last read about monopole history back in high school, thus I already forgot the year when magnetic charge is still the mainstream understanding of what magnets are made of

yeah, just like how people love to quote Einstein and hawking while not really pay attention on what they really have said

misquotes and abuse of authority is way too common in popsci

@Slereah There are several things that you can use to build confidence in the work of others.

1. Social distance. In your own field you are likely connected to the people who did the work through a small number of links (and even better through several completely distinct paths of similar length). It means you can probably a handle on how the authors are seen by their peers.

2. Deep interconnections between fields. For well developed fields there are many connection with seemingly disparate fields. By knowing enough about those deep theories and knowing your own field well, you improve your chances of being able to check a few aspects of another field against stuff you do know.

3. Fields—like particle physics—in which many experiments are highly general see some measurements done over and over again. And that gives you both some touchstone facts and way to evaluate the basic competence of others.

but then how can you be sure that physics isn't a giant conspiracy!

@JohnRennie Hi! Good morning :)

Morning :-)

@JohnRennie Gotta ask you something.

I'm pretty busy this morning. Is it quick?

Yea.

In Problem Solving?

@0celo7 small Roman numerals

Someone upvoted this ????

Hey, if you can generate twice as much steam as is actually possible, then you deserve an upvote.

@DawoodibnKareem I wonder if he's looking for people to invest in his company :-)

So I have Feynman's Theory of Fundamental Process

and it's probably the worst textbook

Oh yes, quite possibly. Sadly, all he's generated so far is hot air.

it's just some dude who took down notes during a Feynman lecture

when he was rambling about QFT

(I mean Virendra Gandhi, not Richard Feynman)

no structure and no details

it's physics at its most physics

Oh that's annoying. I've rushed through this mornings work because I need to take my car to the garage, and the garage has just phoned to cancel the appointment.

Oh well. Make more coffee I guess.

Will they be able to fix it before the upcoming holiday?

Do u mean the death and resurrection of our lord Christ

(HE IS RISEN)

I can't imagine meaning such a thing. But let's not have a religious argument in this room.

I'm up for any holiday with chocolate and bunnies, really

Well fair enough. Chocolate is good. And there's always myxomatosis.

People are full of diseases too, yet I don't avoid them

although really, when I don't have to go among people, I tend to get sick much less often

Oh, I'm not proposing avoiding anything. I don't really object to two paid days off work.

You know what else has bunnies?

This is clearly some new branch of mathematics that I'm unaware of. Looks awfully hare-raising.

It's just group theory but done using Penrose notation by a crazy man

@DawoodibnKareem no, it's now booked for next Tuesday. But that's OK.

So you weren't planning a long Easter journey in it then?

No. I'm down at my Mum's in Somerset. She's been a bit poorly and I've been looking after things. I'm not going home until 10th April.

Oh, I'm sorry to hear she's not well.

*me mum's

@DawoodibnKareem she had a stroke, which is a bit scary but in fact she's recovered really well and is virtually back to what we laughingly refer to as normal :-)

@Slereah me mam's

Fortunately foreigners don't actually know what low class french sounds like

We get to pretend French is a classy language

How do you choose which dialect of French is the low class one?

English is a bastard mongrel of a language. That's what makes it such fun :-)

@DawoodibnKareem smug superiority of Parisians

@Slereah Yes, I figured as much. Blame the printing press.

Actually there was a concerted effort to stamp out dialects!

In the 19th century

I know.

School teachers had the official mission to teach Proper French to the dirty children of the provinces

Not just dialects of French if my understanding is correct, but also Occitan.

@JohnRennie I'm sorry to hear that John :(

@DawoodibnKareem Well it is somewhat related to French

although it is indeed less related than the other langues d'oil

as they are called

(also Breton :p)

Gotta get rid of the dreaded Albion incursion

@Tanuj it was pretty scary at the time, but everything is fine now. Mum is loonier than an atticful of bats, but she was before the stroke so no change there :-)

Well of course. The printing press set the various langues d'oil on the path to assimilation with Parisian French. Those languages that were different enough from Parisian didn't suffer quite as badly, which is why you still have Provencal (yes I know it has a cedilla - I can't be bothered typing one) .

@JohnRennie haha , hope she stays good for years to come.

@JohnRennie The last time I was in Canada, I saw several vending machines with signs that said "no loonies". I thought this was some kind of bizarre discrimination till someone told me what it meant.

Loonies need not apply

@DawoodibnKareem Google, Google, aha :-)

Remember the time before we had a phone with access to the internet at all times?

Those were scary times!

When you had to carry around a physical map to travel

I frequently see people carrying physical maps around my city. We get visits from a lot of cruise ships full of middle-aged Americans.

Back in the days where if you forgot an actor's name in a movie you just had to accept that you couldn't and get on with your life

Remember books?

Yes, I have one on my desk right now!

It's that terrible Feynman book I mentionned

I love Feynman and all, but his books are a mixed bag

although I suspect that's because a lot of his books just started as lecture notes

A former flatmate of mine still has my copy of Surely You're Joking.

I loved Feynman's popular books when I was an impressionable teenager. Now I'm old and jaded I find them a bit irritating.

You're probably not in his target audience.

His actual papers are actually fairly nice

i'm not sure if he ever wrote like a serious book, though

A scientist either writes for their colleagues, or writes for the hoi polloi. I imagine evolutionary biologists probably find Dawkins jarring.

one not aimed at students

@DawoodibnKareem yes, it's not a judgement on the books. The books have inspired generations of enthusiastic young physics nerds and I can't rate them highly enough.

And some of them even end up in this chat room.

Oh wait

His path integral book

is an example of a serious physics book

How does one say "chat room" in French?

un chat

But of course

Miaow

the Académie Française does not approve of this

@JohnRennie pronounced à l'anglaise

I suppose it's not unreasonable for the same spelling to have multiple meanings.

@Slereah shorten the "a" to an "i". Make it a chit room.

there's an Official French Word that nobody uses

Defined by the Académie Française

I was worried that it might be une salle de chat, or something like that, which really sounds like a place to post your cat pictures.

well in the end, isn't it?

I think in the very early days of french internet, it was groupe de discussion

Does the awww room still exist?

but the english word became common usage

Oh, heck, it does. chat.stackexchange.com/rooms/2110/aww

I hadn't been there for years!

judging by the activity rate, neither have most people

Not quite as bad as linguistics.

a lot of stack exchange chats are fairly dead

I tried going on the HSM chat, it's ded

What's HSM?

history of science and mathematics

Has the potential to be interesting, I guess.

To be fair the Physics chat was very quiet at first

Physics SE is far from both of those :p

I'm not sure if we're the most flagged chat room, but if we aren't it's a close contest :-)

I think I'll just buy Synge's book

even for 200 dollaridoos

it's just kind of a weird book

It's filled with things that nobody writes about anymore

and the people that talk about them just reference back to Synge

Damn, I got distracted watching a video of ducklings climbing stairs.

cat going down stairs are better

A couple of weeks back, I built a staircase for some baby pigeons.

It's hard to tell whether that cat is falling, or doing that deliberately.

a bit slowly to be falling

he's just being lazy

imgur.com/gallery/JqgTICr

It looks painful, but I guess if you've got lots of fur, it wouldn't be too bad.

Cats failing jumps is among my favorite genre

@Slereah How I am after waking up from sleep

Enough! :-)

there is never enough

I'd mock those cats more but I probably can't make great jumps either

That one on the toilet wasn't really trying.

Hmm, Chrome doesn't seem to have a way to turn those blasted animations off ...

just talk more, they'll scroll up :p

Actually there is something to do with physics I was wondering about

is it the cat righting reflex

If we allow magnetic charges then you can no longer usefully define the magnetic vector potential because it becomes multivalued.

Are you sure? What's the equivalent with electric charge?

But suppose we allow magnetic charges but not electric charges. Is there then a corresponding electric four potential that is well behaved?

@JohnRennie Depends on the theory you're using

@Slereah Maxwell's equations

Well obviously not if you're using magnetic charge :p

But if you're just using Maxwell equation with magnetic charges, then yes, the magnetic field isn't just rotational

Maxwell's equations allow a magnetic charge, it's just that in the real world that charge is zero.

So I think you'd need two potentials

Why wouldn't you allow electric charges? You want to forbid most of the stuff in the universe?

@JohnRennie Depends how you write it!

@DawoodibnKareem people tend to say you can't have magnetic charge because ... followed by various arguments such as non-covariance of the vector four-potential.

What I am getting at is that the problem is really you can't have both electric and magnetic charge.

I wonder if there's a symmetry breaking event that forces us to have EITHER magnetic charges OR electric charges, but not both. Presumably that particular symmetry broke very soon after the big bang.

With just one form of charge there's no problem - it's just having both that's the problem.

I mean the decomposition of the EM tensor into a magnetic and electric field relies on having the magnetic field having a zero divergence

and the electric field not stemming from any vector current

If you do, I think you should be able to use two EM tensors to define both

So a universe with just magnetic charge would be perfectly consistent. It's just ruled out by experiment not theoretically.

One with electric charges as source

and the other with magnetic charge as source

@JohnRennie Is there a way to tell the difference?

@JohnRennie Well everything is ruled out by experiment :p

that's how we know things are ruled out!

@DawoodibnKareem another good question. In a universe with only magnetic charge could you have magnetically bound atoms? Would the electrons have magnetic charge and electric dipoles instead of electric charge and magnetic dipoles?

That's more or less what I was just typing.

I was imagining atoms made of southtrons around a nucleus of northtrons and neutrons.

@JohnRennie the real question is, if you replace all instances of "electric" with "magnetic", is it really a different theory?

Or are you just switching the $E$ and $B$ label

But struggling to find the right verb for what electrons do.

apparently you can write it easily enough in tensor notations

and with only one tensor

but

you have to use the dual EM tensor

@Slereah so many good questions. So few answers :-)

$\partial_a F^{ab} = j^b$ and $\partial_a \tilde{F}^{ab} = j_m^b$

With $\tilde{F}^{ab} = \dfrac 12 \varepsilon^{abcd}F_{cd}$

Also apparently if you include magnetic charges there's some $U(1)$ gauge symmetry between magnetic and electric charge

Well, not a gauge symmetry

just a symmetry

@Slereah so you can rotate an electric charge into a magnetic charge and vice versa?

Yes

Which goes along with what I was saying about switching labels

Hmm, so there's no deep significance to the non-existence of magnetic charges specifically, because the labels magnetic and electric are arbitrary. The point of physical significance is that we have only one form of charge.

Which may or may not be related to general covariance ...

Well I mean, up and down quarks are also related by a global symmetry, doesn't mean they're the same :p

I think on the other hand, for theories with magnetic charge, you don't have $F = dA$

but I'd have to check

In this answer, what happens to the +C from integration? electronics.stackexchange.com/a/238225

"Since F is no longer a closed form, its expression must be modified by the introduction of a non-exact part, say C "

Yesss

Oh, so actually, the differential forms of Maxwell equations are about the magnetic charge!

Neat

$dF = j$ is that the EM field stems from a charge, while $d \star F = 0$ means that the dual EM field stems from (zero) magnetic charge

So in fact allowing a magnetic charge causes us no problems with covariance. It just adds an extra field?

Well, it's not great either

Because it means that $F$ isn't a closed form anymore

it doesn't change the covariance but it makes things more complicated

I'm not sure what that means from the point of view of gauge theory

Although apparently yes, as I suspected you get two vector potentials

$$F = dA - \star dB$$

sciencedirect.com/science/article/pii/0550321377902218

might be relevant

@Rick The circuit has zero resistance, so it wouldn't matter if there was some extra current in it. So the +c is basically irrelevant.

This is what I love about physics. With a bit of hard work you can get a real understanding of the issues involved. This stands in stark contrast to the pointless ranting we endured last night!

The point is, the maths gets really hard if you don't use complex numbers, and really easy if you do use them. The question asks whether it makes sense physically to use them. That's something you can debate all day, but what matters is that it makes sense pragmatically.

Are there any experiment proposed to detect the dilaton field

@DawoodibnKareem Hmm..but the emf source is $ Vcos(\omega t + \phi)\$ , which means it could start with a non zero value

Doesn't matter.

Heh

There's a Newtonian limit for string theory

@Rick That just means you start measuring stuff at a different time.

@Slereah Did you really think there wouldn't be?

$$\Phi(r) \approx -\frac{G_N M}{r} (1 - e^{-c_1 r}) + c_2 r^2 + c_3 \cosh(c_1 r) $$

Although I'm guessing the exponential part is very short range

@DawoodibnKareem I really didn't get it...if the resistance is zero, current can start with a non-zero value at t=0, so why are we ignoring the +c? are we taking i+c as a new i?

Yes, current can start non-zero in this idealised circuit, and it will just keep flowing.

It doesn't make any difference.

There's nothing dissipating the energy from a constant current. There's only inductance. And what inductance does is it resists changes in current.

ok

Now, an AC current, because it's constantly changing, finds that an inductance acts a lot like a resistance. The energy carried by the current gets dissipated in the inductor.

But a constant DC current doesn't lose energy in an idealised inductor with zero resistance.

When you're doing calculations with AC currents and voltages, you have to treat inductance and capacitance like resistance - except that there's also a phase shift between the voltage and the current. If you use complex numbers to represent both the magnitude and the phase of the voltage and the current, then both inductance and capacitance look like imaginary-valued resistance.

We use the word "impedance" to mean the combination of resistance, inductance and capacitance, with the same dimension as resistance.

arxiv.org/pdf/1208.1433.pdf

neat

And its value is a complex number. Other than that, it's exactly like resistance.

How exactly does the inductor dissipate energy in AC?

You have to scale the inductance and the capacitance by the angular frequency, because that affects how much they dissipate energy.

@Rick I think you get electromagnetic waves out of it.

allaboutcircuits.com/textbook/direct-current/chpt-15/… has good information, but doesn't answer your last question.

Dilaton fields are p. interesting since they can violate the ANEC

Tho I suspect they are pretty mild if they exist

@Rick No, I'm wrong. An inductor stores energy, and you lose it in one part of the AC cycle and gain it in the next.

oh ok

It's explained here electronics-tutorials.ws/inductor/ac-inductors.html

I guess the alternating changes in current mean that the inductor is alternately pushing against the voltage and pushing with the voltage.

Sorry, it was 1989 that I last studied this stuff!

@JohnRennie : "This stands in stark contrast to the pointless ranting we endured last night!" What pointless ranting?

@Slereah : there aren't two potentials, there's only one. See the Wikipedia electromagnetic radiation article and note this: "the curl operator on one side of these equations results in first-order spatial derivatives of the wave solution, while the time-derivative on the other side of the equations, which gives the other field, is first order in time".

@Semiclassical : that one set of field lines was supposed to represent a "spinor". You've seen something similar in depictions of the gravitomagnetic field. Heaviside developed gravitomagnetism as an analogy of electromagnetism.

The Endless

@Slereah Details?

@Semiclassical It's in Jackson, apparently

Lemme see

@JohnRennie @Slereah For magnetic charges and potentials, see physics.stackexchange.com/a/319434/50583 and the answers I link there